Workpiece transfer apparatuses for punch presses of the type including a reciprocating ram actuated against a series of in-line progressively forming die stations for moving workpieces progressively from one die station to the next are well known in the art. For many applications, a transfer apparatus is most effective when actuation is via mechanical linkages responsive to the reciprocation of the press ram. Transfer apparatuses of this type embrace the workpieces from opposite sides and then index them to their respective next die stations as the press ram ascends. The transfer apparatuses retract from the workpieces and reposition themselves to a starting position as the press ram descends.
It will be readily appreciated by those skilled in the art that jamming of the transfer apparatus due to mislocated workpieces, and the like, is a frequent occurrence. The likelihood of a jam, therefore, must be considered in the design of any workpiece transfer apparatus so that when the jam occurs, the transfer apparatus will cease to respond to the press ram movements to prevent damage.
The prior art teaches the use of a safety clutch to disengage workpiece engaging jaws on the transfer apparatus from responding to the press ram movements. Examples of these prior art teachings may be found in U.S. Pat. No. 4,198,845, issued Apr. 22, 1980, and 4,513,602, issued Apr. 30, 1985, both to the inventor of the subject application. These prior art patents disclose mechanically actuated workpiece transfer apparatuses each including a drive rock shaft supported for oscillation in fixed bearings and having one or two rocker arms extending from the drive rock shaft to displace two workpiece engaging jaws. One safety clutch is provided between the two jaws and the drive rock shaft to simultaneously release both jaws from actuation upon a predetermined resistance against the driving force. In other words, the two workpiece engaging jaws, although separate members, are actuated through a common clutch, so that upon the occurrence of a jam, the two jaws simultaneously disengage from further actuation.
As is well known, the single safety clutch of a prior art workpiece transfer apparatus is frequently preset to a minimum threshold release, i.e., disengagement, value so that normal operating resistances against the driving force do not accidentally trip the single clutch. In this example, the threshold value for the safety clutch is set to disengage at a one hundred pound resistance against the driving force. In other words, the single prior art safety clutch will disengage when resistance against the driving force exceeds the one hundred pound threshold value. More often than not, a workpiece jam will only exert resistance against one of the workpiece engaging jaws. This jammed workpiece, therefore, must exert a resistance in excess of the one hundred pound threshold hold value against that one jaw in order to disengage the safety clutch. Damage may result to very expensive equipment when the jammed workpiece is unable to exert the requisite resistance against the one jaw. In these type situations where the jam is not of sufficient magnitude to trip the single safety clutch, damage will inevitably result.